Traditionally, full text retrieval over structure peer-to- peer network has been implemented by inverted index by keywords. However, search based on this index scheme only support literally word match, not taking into account the meaning of words. In this paper, we present a new index scheme, inverted index by concepts, for document retrieval over structure P2P network. We introduce ontology to capture the semantic relations between words, which provide a solution for ambiguity and richness of natural language. The index scheme proposed in this paper transforms the keyword search to the match process of concepts, and implementing keyword search based on semantic. Our index scheme also avoids the intersection of inverted lists between nodes when executing multi-keyword search. Simulation experiment shows that search based on our index scheme has higher retrieval performance than that based on keyword index, and generates lower network traffic.